If Greece has currently (2010) a primary deficit of 4,5% than why would "a default at this time would lead to the cessation of pension and other social payments" and not a net cut of ~5% in all public expenses?

Because the 5% is 5% of GDP, not 5% of government expenses (which are something closer to 35% of GDP - so spending would need to be slashed by 15% across the board.